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Impossible, it was my first IKEA device in my home.
The bulb was still in the bag when I paired the dimmer.
Maybe it was pre-paired in fabric with the bulb? I bought it in a kit (1000lm dimmerable bulb + dimmer).

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Couldnt agree more, that dimmer is s**t. I was able to pair it and then watch my sensors and routers lose connection step by step - removed the dimmer, everything recovers within minutes. Did repeat that for 3 times, then took the battery out - will return it to IKEA.

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edited

SInce this is one of the top results in Google regarding pairing the subject I'll leave here my experience. ( @Koenkk maybe it's worthwhile to add a note into the "pairing" section of the manual. I might be willing to contribute)
I had pretty much the same experience. Including the

The sensor inside HA was created, but it's not alive. It stays in "unknown" state.

However as soon as I have re-started the zigbee2mqtt plugin it just started to work... (I'm using hass.io on VirtualBox).
So, if you are reading these lines trying to make this dimmer work - try to use that good old rule. Restart :)